This invention relates to a common mode rejection coupler for electrically isolating the input of an electrical system from its output.
The difficulty of coupling a low level signal to a circuit having a transient high level common mode voltage is known in the art. In high voltage or high power switching systems, a low level input logic control circuit provides control signals for an output high level power switch control circuit. Typically, when switching occurs, the reference potential of the high level output circuit changes by several hundred volts with respect to the ground potential of the low level control circuit. The potential difference between the floating reference of the high level switching circuit and the ground of the low level control system is termed the inherent common mode voltage. Among techniques known in the art for effectively rejecting common mode voltages while at the same time effectively utilizing low level signals in systems having high common mode voltages include the use of magnetically coupled current transformers and differential amplifiers. These circuits have some limitations particularly because of low frequency response in the case of a transformer coupling, the inapplicability of differential amplifiers to isolate high common mode voltages, and the lack of flexibility when utilizing other systems known in the art.
More recently, as disclosed in Broderick et al., U.S. Pat. No. 3,978,343, and in Loucks, U.S. Pat. No. 4,032,843, optically coupled isolators have been utilized to reject common mode voltages. In its basic form an optically coupled isolator includes a light emitting diode or similar current-to-light transducer which generates light having an intensity which is proportional to the current. The light thus generated is directed to a photo-sensitive device. The photo-sensitive device in turn generates a current which is proportional to the light intensity. Since photons are unaffected by electric fields, an optical coupler has the capability of isolating rather high common mode voltages. Accordingly, these devices have been utilized in solid state switching systems particularly where a low level logic control circuit controls a high power switching circuit.
One principal problem encountered with prior art optical isolator circuits when switching rapidly changing high voltages is that signal energy is capacitively coupled via the stray capacitance between the light emitting diode and the light sensing diode in the electrical circuit. Accordingly, this rapid transition in the common mode voltage can result in the generation of undesirable spurious or transient signals by the optical isolator which can be passed through the system to effect the operation of the isolated switching circuitry.
In the example of power switching systems, the power switching circuitry is controlled by low level logic control circuitry. The reference potential of the high power switching circuitry floats in potential with respect to the low level logic control circuitry such that the common mode voltage can vary in the range of zero to .+-.500 volts in a few microseconds. Because of these high voltage transitions, the stray capacitance between the light emitting diode and the light sensitive diode couples signals which can cause the faulty switching of the high power switching circuitry. There accordingly is a need in the art for a common mode rejection coupler to eliminate spurious signals resulting from transitions in the common mode voltage.